Spectacle ophthalmic lenses are worn and widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness (myopia) and far-sightedness (hypermetropia), astigmatism, and defects in near-range vision usually associated with aging (presbyopia).
Ophthalmologists or optometrists routinely improve the visual acuity by correcting refractive errors of an eye in terms of sphere, cylinder and axis.
Astigmatism of an eye occurs when the refractive error in an eye is meridian-dependent. This is usually due to one or more refractive surfaces, most commonly the anterior cornea, having a toroidal shape.